1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving the device.
2. Description of the Related Art
In the past, TN-mode liquid crystal displays (LCDs) were used often. Recently, however, VA-mode LCDs and IPS-mode LCDs with better viewing angle characteristics than the TN-mode LCDs have been used more and more extensively. Meanwhile, MVA-mode LCDs and S-IPS mode LCDs with further improved viewing angle characteristics have been used for TVs and monitors lately.
The VA mode realizes black display of higher quality than the IPS mode, and therefore, contributes to presenting images at a higher contrast ratio. But the VA mode is inferior to the IPS mode in that the γ characteristic has heavier viewing angle dependence.
The applicant of the present application developed a novel pixel division technique as a means for reducing the viewing angle dependence of the γ characteristic in the VA mode and disclosed it in Japanese Patent Application Laid-Open Publication No. 2004-62146 (Patent Document No. 1) and Japanese Patent Application Laid-Open Publication No. 2004-78157 (Patent Document No. 2). According to this pixel division technique, each pixel is divided into at least two subpixels with mutually different luminances and the γ characteristics of those subpixels are superposed one upon the other, thereby improving the γ characteristic when the screen is viewed obliquely. Specifically, Patent Documents Nos. 1 and 2 disclose the technique of making the luminances of those subpixels different from each other by applying storage capacitor counter voltages through electrically isolated CS bus lines to storage capacitors (CS) that are provided for respective subpixels.
Another pixel division technique was disclosed in Japanese Patent Application Laid-Open Publication No. 2003-295160 (Patent Document No. 3), for example, prior to the filing of Patent Documents Nos. 1 and 2. The LCD disclosed in Patent Document No. 3 has a configuration in which display signal voltages are applied independently of each other to a plurality of subpixels that a single pixel has. That is to say, if each pixel is made up of two subpixels (which will be referred to herein as “first and second subpixels”, respectively), a source bus line for applying a display signal voltage to the first subpixel and another source bus line for applying a display signal voltage to the second subpixel need to be provided separately. That is why if each pixel is divided into two, then the number of source bus lines and the number of source drivers need to be doubled. On the other hand, in the LCDs disclosed in Patent Documents Nos. 1 and 2, a common display signal voltage is applied through the same source bus line to the first and second subpixels, and there is no need to increase the number of source bus lines or source drivers according to the number of divisions.
According to the pixel division technique, a pixel division structure is adopted as schematically shown in FIG. 1(a). Specifically, a single pixel P shown in FIG. 1(b) is divided into two subpixels SP1 and SP2, of which the luminances are made different in a particular range, thereby reducing the viewing angle dependence of the γ characteristic. In the example shown in FIG. 1, each pixel is divided into two. However, the number of divisions does not have to be two but each pixel may be divided into any other number of subpixels.
Patent Document No. 1 discloses that in applying the pixel division technique described in Patent Document No. 1 to a VA-mode LCD, the area ratio of a bright subpixel (e.g., SP1 in the example shown in FIG. 2) to a dark subpixel (e.g., SP2 in FIG. 2) is preferably defined such that the bright subpixel has a smaller area than the dark subpixel to further improve the γ characteristic when the screen is viewed obliquely. If there are two or more bright subpixels and two or more dark subpixels, the combined area of the bright subpixels is compared to the combined of the dark subpixels. It should be noted that a subpixel that has a higher luminance than that realized by its pixel (to be determined by an input video signal, for example) will be referred to herein as a “bright subpixel”, while a subpixel that has a lower luminance that that realized by its pixel a “dark subpixel”.
Patent Document No. 1 also discloses that when a single pixel is divided into a plurality of subpixels with mutually different luminances, those subpixels with intentionally varied luminances are preferably arranged as randomly as possible (i.e., so as not to follow the order of magnitudes of the luminances) such that multiple subpixels with the same degree of luminance are not adjacent to each other in a column direction or in a row direction. Specifically, Patent Document No. 1 gives an example in which a single pixel is divided into two subpixels with a one to one area ratio and in which the bright and dark subpixels are arranged in a checkered pattern. However, this patent document fails to teach how those bright and dark subpixels should be arranged in a situation where the area ratio of the bright subpixels to the dark subpixels is uneven (e.g., one to three).
According to the disclosure of Patent Document No. 1 about a two-subpixel structure in which the area ratio of the bright subpixel to the dark subpixel is one to three, the arrangements shown in FIGS. 3(a), 3(b) and 3(c) in which multiple subpixels with the same magnitude of luminance are adjacent to each other either in the column direction or in the row direction would not be preferred but the checkered pattern shown in FIG. 3(d) should be preferred.
When the present inventors adopted the arrangement shown in FIG. 3(d), however, the resultant image either looked blurred (i.e., with an unclear contour) or generated zigzag lines (i.e., a pseudo contour) in the row direction as shown in FIG. 15(a). These phenomena are observed when the difference in luminance between the bright and dark subpixels changes according to the gray scale to be displayed. And we discovered that those phenomena were caused because the luminance centroid of a pixel shifted with the gray scale to be displayed. It should be noted that the “pixel” refers to herein the minimum display unit of an LCD and corresponds to a “picture element (or dot)” that displays each color (which is typically R, G or B) in a color display device. Also, when the arrangement of pixels is discussed for a color display device, it is pixels to display the same color that should be arranged unless otherwise stated. For example, the three pixels shown in FIG. 3 are pixels to display the same color. As for a physical arrangement in a color display device, pixels (i.e., picture elements or dots) to display other colors will be present between the pixels illustrated.